Thin film magnetic recording disks and disk drives are conventionally employed for storing large amounts of data in magnetizable form. In operation, a typical contact start/stop (CSS) method commences when a data transducing head begins to slide against the surface of the disk as the disk begins to rotate. Upon reaching a predetermined high rotational speed, the head floats in air at a predetermined distance from the surface of the disk where it is maintained during reading and recording operations. Upon terminating operation of the disk drive, the head again begins to slide against the surface of the disk and eventually stops in contact with and pressing against the disk. Each time the head and disk assembly is driven, the sliding surface of the head repeats the cyclic operation consisting of stopping, sliding against the surface of the disk, floating in the air, sliding against the surface of the disk and stopping.
For optimum consistency and predictability, it is necessary to maintain each transducer head as close to its associated recording surface as possible, i.e., to minimize the flying height of the head. Accordingly, a smooth recording surface is preferred, as well as a smooth opposing surface of the associated transducer head. However, if the head surface and the recording surface are too flat, the precision match of these surfaces gives rise to excessive stiction and friction during the start up and stopping phases, thereby causing wear to the head and recording surfaces, eventually leading to what is referred to as a "head crash." Thus, there are competing goals of reduced head/disk friction and minimum transducer flying height.
Conventional practices for addressing these apparently competing objectives involve providing a magnetic disk with a roughened recording surface to reduce the head/disk friction by techniques generally referred to as "texturing." Conventional texturing techniques involve mechanical polishing or laser texturing the surface of a disk substrate to provide a texture thereon prior to subsequent deposition of layers, such as an underlayer, a magnetic layer, a protective overcoat, and a lubricant topcoat, wherein the textured surface on the substrate is intended to be substantially replicated in the subsequently deposited layers. The surface of an underlayer can also be textured, and the texture substantially replicated in subsequently deposited layers.
A typical longitudinal recording medium is depicted in FIG. 1 and comprises a substrate 10, typically an aluminum (Al)-alloy, such as an aluminum-magnesium (Al-Mg)-alloy, plated with a layer of amorphous nickelphosphorus (NiP). Alternative substrates include glass, glass-ceramic materials, plastics, silicon and graphite. Substrate 10 typically contains sequentially deposited on each side thereof a chromium (Cr) or Cr-alloy underlayer 11, 11', a cobalt (Co)-base alloy magnetic layer 12, 12', a protective overcoat 13, 13', typically containing carbon, and a lubricant topcoat 14, 14'. Cr underlayer 11, 11' can be applied as a composite comprising a plurality of sub-underlayers 11A, 11A'. Cr underlayer 11, 11', Co-base alloy magnetic layer 12, 12' and protective overcoat 13, 13', typically containing carbon, are usually deposited by sputtering techniques performed in an apparatus containing sequential deposition chambers. A conventional Al-alloy substrate is provided with a NiP plating, primarily to increase the hardness of the Al substrate, serving as a suitable surface to provide a texture, which is substantially reproduced on the disk surface.
In accordance with conventional practices, a lubricant topcoat is uniformly applied over the protective overcoat to prevent wear between the disk and head interface during drive operation. Excessive wear of the protective overcoat increases friction between the head and disk, thereby causing catastrophic drive failure. Excess lubricant at the head-disk interface causes high stiction between the head and disk. If stiction is excessive, the drive cannot start and catastrophic failure occurs. Accordingly, the lubricant thickness must be optimized for stiction and friction.
A conventional material employed for the lubricant topcoat comprises a perfluoro polyether (PFPE) which consists essentially of carbon, fluorine and oxygen atoms. The lubricant is usually dissolved in an organic solvent applied and bonded to the carbon overcoat of the magnetic recording medium by techniques such as thermal treatment, ultraviolet (UV) irradiation and soaking. A significant factor in the performance of a lubricant topcoat is the bonded lube ratio which is the ratio of the amount of lubricant bonded directly to the carbon overcoat of the magnetic recording medium to the amount of lubricant bonded to itself or to a mobile lubricant. Desirably, the bonded lube ratio should be high to realize a meaningful improvement in stiction and wear performance of the resulting magnetic recording medium.
The escalating requirements for high areal recording density impose increasingly greater requirements on thin film magnetic media in terms of coercivity, stiction, squareness, low medium noise and narrow track recording performance. In addition, increasingly high areal recording density and large-capacity magnetic disks require increasingly smaller flying heights, i.e., the distance by which the head floats above the surface of the disk in the CSS drive (head-disk interface). For conventional media design, a decrease in the head to media spacing increases stiction and drive crash, thereby imposing an indispensable role on the carbon-protective overcoat.
There are various types of carbon, some of which have been employed for a protective overcoat in manufacturing a magnetic recording medium. Such types of carbon include hydrogenated carbon, graphitic carbon or graphite and nitrogenated carbon or carbon nitride. These types of carbon are well known in the art and, hence, not set forth herein in great detail. See, for example, L. J. Huang et al., "Structure of Nitrogenated Carbon Overcoats on Thin Film Hard Disks," IEEE Transaction on Magnetics, Vol. 33, 1997; L. J. Huang et al., "Characterization of the head-disk interface for proximity recording," IEEE Transaction on Magnetics, 1997, Vol. 33, pp. 3112-3114; and Tsai et al., "Character Review Characterization of diamond like carbon films and their application as overcoats on thin-film media for magnetic recording," J. Vac. Sci. Technol., A5(6), Nov/Dec, 1987, pp. 3287-3311.
Generally, hydrogenated carbon has a hydrogen concentration of about 5 at. % to about 40 at. %, typically about 20 at. % to about 30 at. %, and does not bond well to a subsequently applied lubricant topcoat by virtue of the passivation of carbon dangling bonds by hydrogen. Accordingly, it is difficult to effectively bond a lubricant topcoat to a hydrogenated carbon protective overcoat at a suitable thickness. Hydrogenated carbon has a lower conductivity due to the elimination of the carbon band-gap states by hydrogen. Hydrogenated carbon also provides effective corrosion protection to an underlying magnetic layer.
Amorphous carbon nitride, sometimes referred to as nitrogenated carbon, generally has a nitrogen to hydrogen concentration ratio of about 5:20 to about 30:0. Carbon nitride generally has more carbon band-gap states than hydrogenated carbon and, hence, a higher conductivity. In addition, carbon nitride contains more dangling bonds than hydrogenated carbon, which dangling bonds promote interactions between lubricant and carbon and, hence, enable the application of a thicker bonded lubricant topcoat. Graphitic carbon or graphite contains substantially no hydrogen and nitrogen and has less band-gap states vis-a-vis nitrogenated carbon but more band-gap states than hydrogenated carbon.
The drive for high areal recording density and, consequently, reduced flying heights, challenges the limitations of conventional practices in manufacturing a magnetic recording medium containing a carbon protective overcoat. For example, a suitable protective overcoat must be capable of preventing corrosion of the underlying magnetic layer, which is an electrochemical phenomenon dependent upon factors such as environmental conditions, e.g., humidity and temperature. In addition, a suitable protective overcoat must prevent migration of ions from underlying layers into the lubricant topcoat and to the surface of the magnetic recording medium forming defects such as asperities. A protective overcoat must also exhibit the requisite surface polarity to enable bonding thereto of a lubricant topcoat in an adequate thickness. A protective overcoat must also exhibit a suitable electrical conductivity. The absence of conductivity may result in the formation of a static charge on the surface of the magnetic recording medium leading to recording and/or reading errors. Furthermore, as the head disk interface decreases to less than 1 microinch, it is necessary to reduce the thickness of the carboncontaining protective overcoat to below the conventional thicknesses employed, e.g., about 200 .ANG.. It is virtually impossible to satisfy such imposing requirements with a conventional protective overcoat material.
Accordingly, there exists a need for a magnetic recording medium comprising a protective overcoat capable of satisfying the imposing demands for high areal recording density and reduced head disk interface. There also exists a need for a magnetic recording medium having a protective overcoat capable of preventing corrosion of the underlying magnetic layer, preventing migration of ions from underlying layers, providing a suitable surface polarity for adequate lubricant bonding and exhibiting suitable conductivity to avoid reading and/or recording errors. There exists a particular need for such a magnetic recording medium exhibiting improved magnetic recording performance, e.g., high remnant coercivity (Hr) and a high signal to noise ratio (SNR).